Automotive seatbelt systems are of vital importance in occupant protection to reduce the injury severity of the occupant in the crash accidents by restraining the occupants to the seat when accidents occur.
Conventional seatbelts have limitations in occupant protection in the two following aspects:
First, for comfort, there is always some slack in the seatbelt webbing, especially when the clothes are thick. When an accident occurs, the webbing slack delays the time that the seatbelt provides restraint to the occupant and thus reduces its protection or effectiveness.
Second, when the seatbelt is tightened in a severe crash, the seatbelt load applied to the occupant could go beyond the occupant's injury tolerance and thus hurt the occupant.
Because of the limitations mentioned above, pretensioner seatbelts and load limiter seatbelts are more and more widely applied in recent years. In the early stage of a crash, the pretensioner reduces the slack of the seatbelt and lets the occupant into the constriction state as soon as possible; in the process of the crash, the load limiter controls the force applied to the occupants by the seatbelt to an acceptable level.
For pretensioner seatbelt systems, when a crash accident is detected, the pretensioner device is activated. There are several types of pretensioner devices. A popular explosion type of pretensioner device includes a gas initiator, a gas generator, a piston, a tube, a wire and a drive wheel. The piston is installed in the tube. One end of the wire is connected to the piston and the other end is connected with the drive wheel. When the pretensioner is activated, the gas initiator ignites the gas generator, causing the gas expansion and forcing the piston to pull the wire, which rotates the drive wheel. The webbing wound on the retractor driven by the wheel is retracted. At the end of the pretension process, the retractor locks and the seatbelt system restrains the occupants from hitting on the front panel or the steering wheel, etc.
In the existing seatbelt pretensioner designs, the pretensioning stroke distance is equal to the reduction of slack length of the webbing. Studies have confirmed that in crash accidents, if pretensioners can retract more webbing as soon as possible, the seatbelt system can actuate earlier and provide better protection.
The operation procedure of the load limiter seatbelt systems is as follows: when the load applied by the webbing to the occupants exceeds a certain level, the retractor or buckle will release some webbing under a preset load level to prevent an overly high load on the occupants. The main types of the load limiter are compressed deformation of metals, webbing unfolding, tearing deformation of side plate metals and torque deformation, etc.
While the use of pretensioners and load limiters could enhance the crash performance of a seatbelt system in some crashes, the use of both devices in a seatbelt system has not always been a practical choice due to some issues such as high cost, complicated structure and large space for packaging requirement.